1. Field of the Invention
This invention relates to a display apparatus such as a liquid crystal display and an electroluminescent (EL) display, and more particularly to a multi-gray level display apparatus and a method of displaying an image at many gray levels.
2. Description of the Related Art
In recent years, a demand has grown for a display apparatus represented by a liquid display, which can display images not only in high-resolution but also at various gray levels.
For example, an active-matrix display having a plurality of pixels and a plurality of switching elements such as thin-film transistors (TFTs), each provided for one pixel, comprises a plurality of pixel electrodes, a common electrode opposing the pixel electrodes, a cell of liquid crystal composition held between the common electrode, on the one hand, and the pixel electrodes, on the other. Generally, in a liquid crystal display of this type, a drive voltage is applied to selected ones of the pixel electrodes for each one-frame period, thereby to display an image.
In order to display an image at, for example, 64 (=2.sup.6) gray levels, it is necessary to use as many as 64.times.2 different voltage levels. This is because the display needs to be AC-driven for the purpose of preventing deterioration of the liquid crystal composition used.
To provide a drive voltage having 64.times.2 different voltage levels is to increase power consumption in the drive-circuit IC incorporated in the display or the manufacturing cost of the display. Hence, this method of driving the display is not desirable.
Another display-driving method known is pulse-width modulation. In this method, the time for applying a drive voltage to a pixel, i.e., the pulse width of the drive voltage, not the level of the drive voltage, is changed in accordance with the gray level at which to display the pixel.
Pulse-width modulation, however, requires a complex drive circuit and is difficult to be controlled in order to display an image at as many as 64 (=2.sup.6) gray levels.
To solve these problems to display multi-gray level images, frame-rate control (FRC) has been developed recently. This control method controls the number of consecutive frame (F) periods constituting one display period during which a drive voltage at a preset level is applied to the pixel electrode to turn on a pixel. Jpn. Pat. Appln. KOKAI Publication No. 2-115893 discloses a technique of preventing flicker on a liquid crystal display to which the FRC method is applied. In the technique, a block of neighboring pixels is determined as a unit to be controlled, and the number of frame (F) periods constituting one display period is variably set between the pixels to be turned on.
With the above-mentioned FRC method it is not necessary to change the level of the drive voltage, and it is possible to solve the problems inherent in the pulse-width modulation.
To display images at more different gray levels by means of the FRC method, however, it is necessary to further increase the number of frame (F) periods which constitute one display period. If an image is displayed at, for example, 64 (=2.sup.6) gray levels, the number of frame (F) periods will increase so much that the image can no longer be recognized as a multi-gray level one or may be deteriorated with flicker.
The present invention has been made to solve the technical problems described above. The object of the present invention is to provide a display apparatus for displaying multi-gray level images without lowering their quality or causing flicker on them, and also a method of displaying an image at many gray levels.